The 411 on Facebook Places and privacyFacebook just rolled out its take on Foursquare’s "check-in" feature, which lets you tell your friends — or the world, if you like — that you’ve just ambled into a specific venue. The twist? Unlike Foursquare, Facebook Places will let your Facebook pals go ahead and check you into a place, as well — cool if you're a freewheeling social butterfly, but troubling if you're at all protective of your privacy.

Announced and launched last night, Facebook Places is only available to U.S. Facebook users for now (and it hasn't been rolled out to every U.S. Facebook user yet, so be patient), and only on the Facebook iPhone app or on mobile devices with HTML 5-compliant browsers that boast geolocation features.

Here's how it works (this will sound familiar to anyone who's used Foursquare before): If you're out on the town and you want your Facebook friends to know that you've just arrived at the Starbucks down the street, you fire up Facebook on your Places-enabled handheld, tap the Places icon, search for the venue you want to check into (you can add a venue if it isn't listed), and tap the "Check In" button. (You must actually be physically near a place to check in; for example, you can't check into the Louvre if you're sitting at home in Plano, Texas.)

Once you've checked into a place, other Facebook Places users at your location will (for a few hours, at least) be able to see that you've checked in with them, and your check-in will appear in your Facebook news feed.

What sets Facebook Places apart from other Foursquare-like "check-in" apps, however, is that you can check your fellow Facebook users into a place at the same time that you check in yourself, simply by tagging them. So, for instance, if you're strolling into a movie with, say, your friends Pete and Amber, you can check yourself in and check in Pete and Amber as well; all three of you will appear in the "People Here Now" list for that particular theater, and your check-ins will pop up as status updates. (You'll be notified the first time a Facebook friend tries to tag you through Places...)